The liver fatty acid binding protein (L-FABP) of zebrafish is a 14-kD cytoplasmic protein that binds long-chain fatty acids (LCFAs) with high affinity. The putative functions assigned to L-FABP include the desorption of LCFAs from the plasma membrane to the cytoplasm, the promotion of intracellular fatty acid (FA) diffusion, the targeting of FAs to different metabolic pathways, and protection against the cytotoxic effects of free FA. Three FABP types have been found in zebrafish organs/tissues: intestinal-type FABP (I-FABP), brain-type FABP (B-FABP), and liver-type FABP (L-FABP). The zebrafish FABPs were originally named according to their site of initial isolation. The zebrafish I-FABP is uniformly expressed throughout the intestine. The zebrafish B-FABP mRNA is expressed in the periventricular gray zone of the optic tectum of the adult zebrafish brain. The L-FABP is expressed exclusively in the liver of the adult zebrafish.
FABPs are found in other vertebrates as well, for example, in mice, rats and humans. Three homologous genes encode FABPs in mice: mouse liver fatty acid-binding protein (L-FABP, or Fabpl), intestinal fatty acid-binding protein (I-FABP, or Fabpi), and ileal lipid binding protein (Ilbp). Mouse, rat, and human L-FABP are transcribed in the liver (hepatocytes) and intestines (postmitotic, differentiating members of the enterocytic lineage), in contrast to zebrafish, in which L-FABP is expressed solely in the liver. The study of mouse and rat L-FABP has been used as a model for understanding the mechanisms that determine distinct regional expression along the gut tube, as well as within the liver. As in zebrafish, L-FABP is thought to play a pivotal role in other vertebrates in the intracellular binding and trafficking of fatty acids in the liver. The importance of L-FABP in vertebrate physiology is underscored by the fact that L-FABP mRNA constitutes 1.6% of translatable RNA of adult male rat livers and accounts for 3 to 5% of the cytosolic protein mass in rat hepatocytes.
Zebrafish have been used extensively to study vertebrate embryonic development, yielding insights into the formation and function of individual tissues, organ systems and neural networks. Transgenic zebrafish, which express transgenes under the control of either zebrafish or heterologous expression control sequences, have been particularly useful in this regard. Zebrafish comprising transgenes, mutant genes, or genes whose expression is altered in some other fashion, can also serve as model systems for diseases in other vertebrates, including humans, and can provide insight into disease mechanisms. Review articles summarizing the use of zebrafish as disease models include Shin et al. (2002), Ann Rev Genomics Hum Genet 3, 311-40; Grunwald et al. (2002), Nature Reviews/Genetics 3, 717-724; Briggs et al. (2002), Am J Physiol Regulatory Comp Physiol 282, R3-R9; Zon (1999), Genome Research 9, 99-100; and Amatruda et al. (2002), Cancer Cell 1, 229-231, which are herein incorporated by reference.
In view of similarities in liver function and development between zebrafish and other vertebrates, it is expected that mutant zebrafish, including transgenic zebrafish, could serve as models for pathological studies of the liver in other vertebrates, including humans. At approximately 32 hpf, the zebrafish liver derives from the primitive gut tube as a morphologically distinct left ventrolateral diverticulum. Like its mammalian counterpart, the zebrafish liver produces bile, which is evident by 3 dpf under the dissecting microscope. Several zebrafish mutations with early liver degeneration have been isolated. For example, the lumpazi, gammler, and tramp mutations encode defects at three loci that lead to liver necrosis. The beefeater mutation shows liver necrosis and impaired glycogen utilization, as seen in the human glycogen storage diseases. Many different types of hepatic injury—e.g., alcohol, infection, and toxins—cause a similar pattern of histological degeneration and ultimately lead to cirrhosis. The pathways leading to massive liver failure are presently poorly understood. The only remedy currently available at such late stages in humans is transplantation of the liver.
Studies with zebrafish, particularly transgenic zebrafish, in which reporter genes are driven by liver-specific expression control sequences, would be useful for, e.g., the study of pathways involved in liver morphogenesis, for the study of disease conditions involving liver pathology, and as the basis for assays for modulatory agents, such as drug candidates or environmental mutagens.